1. Field of the Invention
The present invention relates to a thick-film integrated circuit device having a substrate formed from an insulating material, as sheet glass, containing alkali-metal compounds, and circuit patterns formed on the substrate.
2. Description of the Prior Art
In the thick-film circuit device, conductive materials are printed and fired on the substrate of insulating material by the screen printing method to form wiring conductors thereon. In addition, in order to increase the packaging density of circuits as necessary, an insulating layer (hereinafter referred to as a crossover) for interlayer insulation is printed and fired on the above-mentioned wiring conductors so as to make multilayer interconnection possible. Thus, electric circuits are formed on a single sheet of substrate.
In the thick-film circuit device, as mentioned above, electronic circuits can be formed by repeating such processes as printing and firing. In this manner, the production process can be simplified and the device in which the electronic circuits are incorporated can be miniaturized because the packaging density of the circuits can be made high. Thus, the thick film production process have become widely used for producing electronic circuits of various devices.
For instance, in the case of fluorescent display tubes in which thermions emitted from the filamentary cathode heated are made to impinge on phosphor-coated anodes thereby to visually display numerals, characters, graphic forms and the like, those of the flat type or the multi-digit type have become easy to produce by the introduction of the thick-film circuit technique.
FIG. 1 shows a perspective view of the substrate and its vicinity of the fluorescent display tube by way of example, and FIG. 2 is an enlarged sectional view thereof. Reference numeral 1 designates a substrate of insulating material. On this substrate 1, wiring conductors 2 are printed and fired according to a desired pattern by the screen printing method. In addition, a crossover 3 having through-holes for the wiring conductors 2 are printed and fired on the substrate 3 at predetermined positions. Furthermore, phosphor-coated anodes 4 are formed on the crossover 3 so that they are electrically connected to the corresponding wiring conductors 2 through the above-mentioned through-holes. External terminals 5 are connected to the ends of the respective wiring conductors 2 so that drive signals are given externally. With the above-mentioned construction, the fluorescent display tube can be easily made to be flat or of the multi-digit type by the introduction of the thick-film circuit technique.
Meanwhile, the substrate for use in the thick-film circuit is heretofore usually made of the so-called porcelain type ceramics such as alumina porcelain. However, such ceramics are inherently very expensive. Therefore, these ceramics have been replaced by sheet glass much more inexpensive than the ceramics.
Recently, the thick-film circuit devices are incorporated in various electronic apparatuses, being used under various environments. Under these circumstances, it has been found that after used for a long time at an elevated temperature the above-mentioned glass substrate tends to undergo deterioration in the insulation between the wiring conductors.
The inventors of the present invention have studied on the cause of the above-mentioned deterioration of insulation, and as a result have come to the following conclusion:
As mentioned above, the substrate for use in the thick-film circuit device is usually made of sheet glass, because it is inexpensive and easy to form. This sheet glass is soda-lime glass containing SiO.sub.2 and Na.sub.2 O as its main alkali component. Meanwhile, the crossover for interlayer insulation must have a firing temperature lower than the softening temperature of the sheet glass used as the substrate, and in addition must have a coefficient of thermal expansion similar to that of the substrate of sheet glass. For this reason, the crossover is usually made of insulating material consisting of lead-borate type glass mainly containing PbO. Thus, the crossover is formed on the substrate by printing and firing the above-mentioned insulating material thereon.
More detailed description will be made on this point with reference to FIG. 2. Sodium ions Na.sup.+ freed from the net-like bonding structure of the sheet glass are dispersed toward the crossover 3 under the influence of high temperatures during the firing process through the interface between the substrate 1 and the crossover 3 and between the wiring conductors 2.
In this state where such sodium ions Na.sup.+ are present, if a potential difference is given between the wiring conductors 2, the above-mentioned sodium ions Na.sup.+ move toward the wiring conductors 2 negative in potential, where they discharge to form sodium Na. Since sodium Na thus produced is active, it reduces PbO present in the crossover 3 to deposit lead (Pb) according to the following reaction: EQU 2Na+PbO.fwdarw.Pb+Na.sub.2 O
The lead Pb thus produced grows dendritically on the wiring conductors 2 negative in potential as shown in a circle designated by character A in FIG. 3, and thereby reduces the insulation resistance between the wiring conductors 2 or causes insulation deterioration therebetween.
The movement of the above-mentioned sodium ions Na.sup.+ becomes more active as the temperature increases. Therefore, if the thick-film circuit device of this type is kept at a high temperature for a long time while being energized, it undergoes deterioration of insulation and decrease in life.
In addition, in the above-mentioned fluorescent display tube, the above-mentioned substrate on which various electrodes are formed are sealed in a vacuum package so as to keep the electrodes in a high vacuum. In the case of a flat-type fluorescent display tube, for instance, a front bulb made of glass or the like is airtightly bonded to the substrate along the periphery thereof to form a vacuum package. In this case, external terminals through which drive signals are externally given to the respective electrodes are airtightly penetrated through the sealing section between the front bulb and the substrate. The sealing section is bonded together using a sealing agent, as frit glass, containing PbO. If a potential difference is given between the external terminals at high temperatures, sodium ions Na.sup.+ dispersed from the substrate or the front bulb to the sealing section are converted into metallic sodium Na. Thus, the metallic sodium Na reduces PbO to deposit metallic lead Pb and thereby deteriorates insulation between the external terminals.